A quick plug for a great company I like a lot (and buy from) is in order. LibertyManiacs.com, makers of "Freedom Products for Liberty Lovers", makes lots of fun and creative designs for t-shirts, posters, buttons, bumper stickers, hats, mugs, and just about anything you might want to slap a Ron Paul R[3VOL]UTION logo on. When you buy via Zazzle.com, it's also really easy to completely customize your item. I recently bought one of their Tank Man t-shirts. I specified that the t-shirt be pale blue, and even added my own text. The original said "Individualism" (and others say things like "Stop Something" and "Disobey"), but I wanted mine to say "Freedom Fighter". The finished product looks great, and I love letting my colors show when I wear it!
"Like" them on Facebook and follow them on Twitter @libertymaniacs so you know when they have discount codes and new designs.